Photovoltaic devices including MG-doped semiconductor films

ABSTRACT

A photovoltaic cell can include a dopant in contact with a semiconductor layer. The photovoltaic cell can include a transparent conductive layer and a first semiconductor layer in contact with the transparent conductive layer, the first semiconductor layer including magnesium. In certain circumstances, a substrate can be a glass substrate. In other circumstances, a substrate can be a metal layer. The first semiconductor layer can include CdS. The first semiconductor layer can have a thickness of between about 200 or 3000 Angstroms. The first semiconductor layer can include 1-20% magnesium. A method of manufacturing a photovoltaic cell can include providing a transparent conductive layer and depositing a first semiconductor layer in contact with the transparent conductive layer, the first semiconductor layer treated with magnesium.

CLAIM FOR PRIORITY

This application is a divisional application of U.S. application Ser.No. 12/507,793, filed Jul. 22, 2009, which claims the priority under 35U.S.C. §119(e) to Provisional U.S. Patent Application Ser. No.61/083,325, filed on Jul. 24, 2008, which are hereby incorporated byreference.

TECHNICAL FIELD

This invention relates to photovoltaic cells.

BACKGROUND

During the fabrication of photovoltaic devices, layers of semiconductormaterial can be applied to a substrate with one layer serving as awindow layer and a second layer serving as the absorber layer. Thewindow layer can allow the penetration of solar energy to the absorberlayer, where the optical energy is converted into electrical energy.Some photovoltaic devices can use transparent thin films that are alsoconductors of electrical charge. The conductive thin films can be atransparent conductive oxide (TCO), such as fluorine-doped tin oxide,aluminum-doped zinc oxide, or indium tin oxide. The TCO can allow lightto pass through a substrate window to the active light absorbingmaterial and also serves as an ohmic contact to transport photogeneratedcharge carriers away from the light absorbing material. A back electrodecan be formed on the back surface of a semiconductor layer. The backelectrode can include electrically conductive material, such as metallicsilver, nickel, copper, aluminum, titanium, palladium, or any practicalcombination thereof, to provide electrical connection to thesemiconductor layer. The back electrode can be a semiconductor material.Doping the semiconductor layer can improve the efficiency of aphotovoltaic device.

SUMMARY

In general, a photovoltaic cell can include a transparent conductivelayer and a first semiconductor layer in contact with the transparentconductive layer, the first semiconductor layer including magnesium. Incertain circumstances, a substrate can be a glass substrate. In othercircumstances, a substrate can be a metal layer.

The first semiconductor layer can include CdS. The first semiconductorlayer can have a thickness of between about 200-3000 Angstroms. Thefirst semiconductor layer can include 1-20% magnesium. In somecircumstances, a transparent conductive layer can be positioned over asubstrate. In some circumstances, a transparent conductive layer can bepositioned over the first semiconductor layer. In some circumstances,the photovoltaic cell can further include a second semiconductor layerin contact with the first semiconductor layer. The second semiconductorlayer can include CdTe. The second semiconductor layer can includecadmium chloride-treated CdTe.

A method of manufacturing a photovoltaic cell can include providing atransparent conductive layer and depositing a first semiconductor layerin contact with the transparent conductive layer, the firstsemiconductor layer treated with magnesium.

The second semiconductor layer can be treated with cadmium chloride. Thesecond semiconductor layer can be treated with heat at approximately380-450 degrees Celsius.

A system for generating electrical energy can include a transparentconductive layer, a first semiconductor layer in contact with thetransparent conductive layer, the first semiconductor layer treated withmagnesium, a first electrical connection connected to a transparentconductive layer, and a second electrical connection connected to a backmetal electrode adjacent to a second semiconductor layer. The firstsemiconductor layer can include CdS. The first semiconductor layer caninclude 1-20% magnesium. In some circumstances, a transparent conductivelayer can be positioned over a substrate. The transparent conductivelayer can be positioned over the first semiconductor layer. In somecircumstances, the photovoltaic cell can further include a secondsemiconductor layer in contact with the first semiconductor layer. Thesecond semiconductor layer can include CdTe. The second semiconductorlayer can include cadmium chloride-treated CdTe.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic of a substrate with multiple layers.

FIG. 2 is a schematic of a substrate with multiple layers.

FIG. 3 is a schematic of a substrate with multiple layers.

FIG. 4 is a graph showing the effect of increasing Mg concentration.

FIG. 5 is a graph showing the effect of CdCl₂ treatment temperature.

DETAILED DESCRIPTION

A photovoltaic cell can include a transparent conductive layer on asurface of the substrate and a first semiconductor layer includingmagnesium. In general, treatment with magnesium allows for a thinnersemiconductor layer to be used, more robust heat treatment conditions,and improved device performance.

Referring to FIG. 1, a photovoltaic cell can include a firstsemiconductor layer 100 can include CdS, for example. The firstsemiconductor layer, such as a CdS layer, can be treated with magnesium101. The CdS layer can include 1-20% magnesium, for example. Optionally,another dopant 102 can also be added to treat the layer. The transparentconductive layer can include a transparent conductive oxide (TCO). Themagnesium or other dopant can be supplied by a source such as a carriergas or by diffusion, either from the substrate 130, from the TCO 120itself, or from a surface layer 140 on the TCO. A second semiconductorlayer 150 can be deposited over a first semiconductor layer. A secondsemiconductor layer can include CdTe, for example.

Referring to FIG. 2, a first semiconductor layer, such as a CdS layer,can be treated with magnesium 201 or other dopant. The magnesium orother dopant can supplied by a source such as a carrier gas or bydiffusion, either from the substrate 230, from the TCO 220 itself, orfrom a surface layer on the TCO. The dopant can form a layer 202 betweena first semiconductor layer 200 a and an additional first semiconductorlayer 200 b. A layer containing a dopant can include 1-20% magnesium,for example. The first semiconductor layer 200 a can have a thicknessgreater than a thickness of the additional first semiconductor layer 200b. For example, the thickness of the first semiconductor layer can begreater than 200 Angstroms, greater than greater than 400 Angstroms, andgreater than 800 Angstroms, or approximately 900 Angstroms. Thethickness of the additional first semiconductor layer can be greaterthan 50 Angstroms, greater than 75 Angstroms, or approximately 100Angstroms. A second semiconductor layer 250 can be deposited over afirst semiconductor layer. A second semiconductor layer can be treatedwith magnesium or alternatively, with another dopant. A secondsemiconductor layer can include CdTe, for example. A dopant layer canhave a thickness greater than 4 Angstroms, greater than 8 Angstroms,greater than 12 Angstroms, or approximately 15 Angstroms, for example.

Referring to FIG. 3, a photovoltaic device can include a firstsemiconductor layer 300 treated with magnesium 301 and a secondsemiconductor layer 350. The first semiconductor layer can be supportedby a substrate 330. Optionally, the second semiconductor layer can betreated with another dopant 302. The first and second dopants can beadded intentionally as extrinsic dopants.

Typically, photovoltaic cell efficiency depends on at least threeparameters: open-circuit voltage, short-circuit density, and fillfactor. Open-circuit voltage (Voc) is the voltage for which the currentin the external circuit is zero. Short-circuit current density (Jsc) isthe current density that flows out of the solar cell at zero bias. Thefill factor (FF) is the ratio of the maximum power point divided by theopen circuit voltage (Voc) and the short circuit current (Jsc).

Increased Jsc results in increased photovoltaic cell efficiency. Jsc canbe increased by reducing the thickness of a semiconductor layer, such asa CdS layer. However, if the thickness of a CdS layer is reduced below acertain limit, the Voc can be adversely affected, resulting in decreasedcell efficiency. Conventionally, high resistive oxide layers between theTCO and CdS layers have been used to overcome this problem. Yet, evenwith high resistive oxides, a CdS thickness of approximately 1000Angstroms is typically required to make effective CdS/CdTe junctions forhigh efficiency cells.

Processing of photovoltaic cells typically involves heat treatment ofCdTe coated plates with cadmium chloride. Cadmium chloride can beapplied by various techniques, such as by solution spray, vapors, oratomized mist. CdTe can also be applied by various techniques, such aseletrodeposition, sputtering, close spaced sublimation, screen printing,evaporation, and vapor transport deposition. A heat treatmenttemperature of 380-450 degrees Celsius is generally most effectiveregardless of the CdTe deposition technique. Generally, temperatureslower than 380 degrees can result in photovoltaic cells with reducedinitial Voc and FF. Temperatures higher than 450 degrees can also resultin photovoltaic cells with reduced Voc and FF, but with a higher Jsc.The increase in Jsc is generally attributed to the diffusion of CdS intoa CdTe layer, which can decrease the effective CdS thickness, therebyfurther reducing Voc.

Applicants have discovered that a modified doping of CdS, such as bydoping with magnesium, can overcome Voc loss due to a thinner CdS layer.Magnesium-doping of CdS can also reduce device sensitivity to variationin CdCl₂ treatment temperatures and reduce the loss of deviceperformance at higher CdCl₂ temperatures.

As shown in Table 1, experiments have shown that treatment of cadmiumsulfide with magnesium (CdS:Mg), when used in CdTe photovoltaic cells,exhibited the following results, indicating that CdS thickness can bereduced, thereby permitting increase Jsc without a loss of Voc. Higherheat treatments can also be applied without resulting in a reduced Voc.

TABLE 1 control magnesium treatment CdS thickness ~1000 Angstroms500-700 Angstroms required without losing Voc Heat permitted 395-410degrees Celsius 430-450 degrees Celsius without reducing Voc

Referring to FIG. 4 and FIG. 5, photovoltaic cells treated having amagnesium-treated CdS layer proved to be more robust against cadmiumchloride treatment temperature variations when compared to cells with anundoped CdS layer. In addition, photovoltaic cells having amagnesium-treated CdS layer achieved higher average Voc than cells withundoped CdS layers. Further, among photovoltaic cells having a CdS layertreated with magnesium, an increased concentration of magnesium wasdirectly related to an increased Voc.

Photovoltaic cells are often processed without any intentional extrinsicdopants in a first semiconductor layer, such as a CdS containing layer,or without any extrinsic dopants in a second semiconductor layer, suchas CdTe layer, or both. Doping the first semiconductor layer withmagnesium can reduce photovoltaic cell instability and increaseefficiency.

Doping a semiconductor layer can be performed in several ways. Forexample, a dopant can be supplied from an incoming powder, such as amixed powder, a carrier gas, or a directly doped powder such as CdSpowder. The powder can be a single or multiple phase material. In othercircumstances, a dopant can be supplied by diffusion from a layer, suchas a substrate layer, a transparent conductive layer, or a semiconductorlayer. A dopant may be added throughout a semiconductor layer, orthrough a portion of a layer.

A dopant for a first semiconductor layer, such as CdS, can be added bydiffusion from the transparent conductive layer or from a surface layeron the transparent conductive layer. Alternatively, a dopant for thefirst semiconductor layer can be added by an incoming powder or carriergas fed into a vapor transport deposition system. A dopant for a firstsemiconductor layer can be volatilized along with the firstsemiconductor at temperatures less than 1500° C., less than 1400° C.,less than 1200° C., less than 1000° C., or less than 800° C., forexample. The volatile species can be atomic or a sulfide molecule, suchas In₂S. Indium can be an effective dopant because it has a reasonablyvapor pressure at typical CdS distributor temperatures (approximately1100° C.).

Doping a second semiconductor layer, such as CdTe, can be performed inseveral ways. For example, a dopant can be supplied along with a CdTesource powder into a vapor transport deposition (VTD) distributorsystem. The dopant for a second semiconductor layer can be introduced asa powder (i.e. a single or multiple phase material) or in a carrier gas.Alternatively, a dopant for a second semiconductor layer can beintroduced by deposition of an outer layer onto the second semiconductorlayer. For example, an outer layer may contain a dopant, which candiffuse onto a CdTe surface.

P-type doping can be effective for a second semiconductor layer, such asCdTe. Group V (for example, N, P, As, Sb, or Bi) and Group I (forexample, Li, Na, K, Rb, Cs, Cu, Ag, or Au) elements can be effectivedopants for a second semiconductor layer. Phosphorous (P), arsenic (As),antimony (Sb), sodium (Na), potassium (K), rubidium (Rb) present novaporization problems when introduced into the VTD system operating attypical CdTe distributor temperatures (approximately 900-1100° C.).

A dopant for a second semiconductor layer can be introduced during heattreatment. Heat treatment can be performed with CdCl₂ flux. Group I andGroup V species such as chloride compounds, can be added to the fluxsolution that is applied to an outer surface of the second semiconductorlayer prior to heat treatment. During heat treatment, recrystallizationcan occur, thereby making doping distribution within the film possible.

Dopant incorporation can be affected by the concentration of vacancydefects. Vacancy concentration can be controlled by vapor phaseoverpressure. For example, excess Cd or Te can be applied to alterelectronic defects in the final device. Cd or Te overpressure can beachieved by introduction of excess Cd or Te in the CdTe source powder.This can be achieved by a two-phase powder blend or by anoff-stoichiometric source material. A material can be off-stoichiometricby greater than 5%, greater than 10%, or greater than 15%, such asapproximately 20%.

A dopant can be applied in any concentration, for example, greater than0.5 ppma, greater than 50 ppma, greater than 500 ppma, or between 1-1000ppma, for example. Certain gas source dopants, such as those containingnitrogen, for example, may decompose too readily in a heated CdTedistributor, and thus, can be introduced in a secondary, lowertemperature distribution system. Dopants can be added throughout anentire semiconductor layer, or into only a portion of the layer by usingmulti-layer VTD configuration, for example.

A first semiconductor layer, such as a CdS layer, can be treated with adopant such as magnesium in several ways, for example, by diffusion fromthe transparent conductive layer or from a surface layer on thetransparent conductive layer. Alternatively, magnesium for the firstsemiconductor layer can be added by an incoming powder or carrier gasfed into a vapor transport deposition system.

A second semiconductor layer, such as a CdTe layer, can also be treatedwith a dopant such as sodium in several ways. In one example, a CdTelayer was doped with sodium using Na₂S powder and Rb. The addition of Nasuggested resulted in changes in recrystallization during subsequentchloride heat treatment, suggesting that Na doping made the CdCl₂recrystallization flux more active.

A CdTe layer can also be doped with P, As, Sb, Na, Rb, or Cu. In yetanother example, a CdTe layer doped with Cu was shown to exhibitimproved electrical signals. The addition of extra Te to the CdTe powderresulted in improved ability to dope the films. In addition, Cd-richconditions resulted in a change in film grain structure suggesting theimproved ability to dope the films.

A common photovoltaic cell can have multiple layers. The multiple layerscan include a bottom layer that is a transparent conductive layer, acapping layer, a window layer, an absorber layer and a top layer. Eachlayer can be deposited at a different deposition station of amanufacturing line with a separate deposition gas supply and avacuum-sealed deposition chamber at each station as required. Thesubstrate can be transferred from deposition station to depositionstation via a rolling conveyor until all of the desired layers aredeposited. Additional layers can be added using other techniques such assputtering. Electrical conductors can be connected to the top and thebottom layers respectively to collect the electrical energy producedwhen solar energy is incident onto the absorber layer. A top substratelayer can be placed on top of the top layer to form a sandwich andcomplete the photovoltaic cell.

The bottom layer can be a transparent conductive layer, and can be, forexample, a transparent conductive oxide such as tin oxide or tin oxidedoped with fluorine. Deposition of a semiconductor layer at hightemperature directly on the transparent conductive oxide layer canresult in reactions that negatively impact of the performance andstability of the photovoltaic device. Deposition of a capping layer ofmaterial with a high chemical stability (such as silicon dioxide,dialuminum trioxide, titanium dioxide, diboron trioxide and othersimilar entities) can significantly reduce the impact of these reactionson device performance and stability. The thickness of the capping layershould be minimized because of the high resistivity of the materialused. Otherwise a resistive block counter to the desired current flowmay occur. A capping layer can reduce the surface roughness of thetransparent conductive oxide layer by filling in irregularities in thesurface, which can aid in deposition of the window layer and can allowthe window layer to have a thinner cross-section. The reduced surfaceroughness can help improve the uniformity of the window layer. Otheradvantages of including the capping layer in photovoltaic cells caninclude improving optical clarity, improving consistency in band gap,providing better field strength at the junction and providing betterdevice efficiency as measured by open circuit voltage loss. Cappinglayers are described, for example, in U.S. Patent Publication20050257824, which is incorporated by reference in its entirety.

The window layer and the absorbing layer can include, for example, abinary semiconductor such as group II-VI, III-V or IV semiconductor,such as, for example, ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgO,MgS, MgSe, MgTe, HgO, HgS, HgSe, HgTe, MnO, MnS, MnTe, AlN, AlP, AlAs,AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb,or mixtures thereof. An example of a window layer and absorbing layer isa layer of CdS coated by a layer of CdTe. A top layer can cover thesemiconductor layers. The top layer can include a metal such as, forexample, aluminum, molybdenum, chromium, cobalt, nickel, titanium,tungsten, or alloys thereof. The top layer can also include metal oxidesor metal nitrides or alloys thereof.

Deposition of semiconductor layers in the manufacture of photovoltaicdevices is described, for example, in U.S. Pat. Nos. 5,248,349,5,372,646, 5,470,397, 5,536,333, 5,945,163, 6,037,241, and 6,444,043,each of which is incorporated by reference in its entirety. Thedeposition can involve transport of vapor from a source to a substrate,or sublimation of a solid in a closed system. An apparatus formanufacturing photovoltaic cells can include a conveyor, for example aroll conveyor with rollers. Other types of conveyors are possible. Theconveyor transports substrate into a series of one or more depositionstations for depositing layers of material on the exposed surface of thesubstrate. Conveyors are described in provisional U.S. application Ser.No. 11/692,667, which is hereby incorporated by reference.

The deposition chamber can be heated to reach a processing temperatureof not less than about 450° C. and not more than about 700° C., forexample the temperature can range from 450-550° C., 550-650° C.,570-600° C., 600-640° C. or any other range greater than 450° C. andless than about 700° C. The deposition chamber includes a depositiondistributor connected to a deposition vapor supply. The distributor canbe connected to multiple vapor supplies for deposition of various layersor the substrate can be moved through multiple and various depositionstations with its own vapor distributor and supply. The distributor canbe in the form of a spray nozzle with varying nozzle geometries tofacilitate uniform distribution of the vapor supply.

The bottom layer of a photovoltaic cell can be a transparent conductivelayer. A thin capping layer can be on top of and at least covering thetransparent conductive layer in part. The next layer deposited is thefirst semiconductor layer, which can serve as a window layer and can bethinner based on the use of a transparent conductive layer and thecapping layer. The next layer deposited is the second semiconductorlayer, which serves as the absorber layer. Other layers, such as layersincluding dopants, can be deposited or otherwise placed on the substratethroughout the manufacturing process as needed.

The transparent conductive layer can be a transparent conductive oxide,such as a metallic oxide like tin oxide, which can be doped with, forexample, fluorine. This layer can be deposited between the front contactand the first semiconductor layer, and can have a resistivitysufficiently high to reduce the effects of pinholes in the firstsemiconductor layer. Pinholes in the first semiconductor layer canresult in shunt formation between the second semiconductor layer and thefirst contact resulting in a drain on the local field surrounding thepinhole. A small increase in the resistance of this pathway candramatically reduce the area affected by the shunt.

A capping layer can be provided to supply this increase in resistance.The capping layer can be a very thin layer of a material with highchemical stability. The capping layer can have higher transparency thana comparable thickness of semiconductor material having the samethickness. Examples of materials that are suitable for use as a cappinglayer include silicon dioxide, dialuminum trioxide, titanium dioxide,diboron trioxide and other similar entities. Capping layer can alsoserve to isolate the transparent conductive layer electrically andchemically from the first semiconductor layer preventing reactions thatoccur at high temperature that can negatively impact performance andstability. The capping layer can also provide a conductive surface thatcan be more suitable for accepting deposition of the first semiconductorlayer. For example, the capping layer can provide a surface withdecreased surface roughness.

The first semiconductor layer can serve as a window layer for the secondsemiconductor layer. The first semiconductor layer can be thinner thanthe second semiconductor layer. By being thinner, the firstsemiconductor layer can allow greater penetration of the shorterwavelengths of the incident light to the second semiconductor layer.

The first semiconductor layer can be a group II-VI, III-V or IVsemiconductor, such as, for example, ZnO, ZnS, ZnSe, ZnTe, CdO, CdS,CdSe, CdTe, MgO, MgS, MgSe, MgTe, HgO, HgS, HgSe, HgTe, MnO, MnS, MnTe,AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, TlN,TlP, TlAs, TlSb, or mixtures or alloys thereof. It can be a binarysemiconductor, for example it can be CdS. The second semiconductor layercan be deposited onto the first semiconductor layer. The secondsemiconductor can serve as an absorber layer for the incident light whenthe first semiconductor layer is serving as a window layer. Similar tothe first semiconductor layer, the second semiconductor layer can alsobe a group II-VI, III-V or IV semiconductor, such as, for example, ZnO,ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgO, MgS, MgSe, MgTe, HgO, HgS,HgSe, HgTe, MnO, MnS, MnTe, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb,InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb, or mixtures thereof.

The second semiconductor layer can be deposited onto a firstsemiconductor layer. A capping layer can serve to isolate a transparentconductive layer electrically and chemically from the firstsemiconductor layer preventing reactions that occur at high temperaturethat can negatively impact performance and stability. The transparentconductive layer can be deposited over a substrate.

A number of embodiments have been described. Nevertheless, it will beunderstood that various modifications may be made without departing fromthe spirit and scope of the invention. For example, the semiconductorlayers can include a variety of other materials, as can the materialsused for the buffer layer and the capping layer. Accordingly, otherembodiments are within the scope of the following claims.

What is claimed is:
 1. A method of manufacturing a photovoltaic cellcomprising: providing a transparent conductive layer; and depositing afirst semiconductor layer comprising CdS and having a first side that isdoped with magnesium, the first semiconductor layer being deposited suchthat the first side is in direct physical contact with the transparentconductive layer.
 2. The method of claim 1, wherein first semiconductorlayer includes 1-20% magnesium.
 3. The method of claim 1, wherein thetransparent conductive layer is positioned over a substrate.
 4. Themethod of claim 1, wherein the transparent conductive layer ispositioned over the first semiconductor layer.
 5. The method of claim 1,further comprising a second semiconductor layer in contact with thefirst semiconductor layer.
 6. The method of claim 5, wherein the secondsemiconductor layer comprises CdTe.
 7. The method of claim 5, furthercomprising treating the second semiconductor layer with cadmiumchloride.
 8. The method of claim 7, further comprising treating thesecond semiconductor layer with heat at approximately 380-450 degreesCelsius.
 9. A method of manufacturing a photovoltaic cell comprising:providing a transparent conductive layer over a substrate; depositing alayer comprising CdS such that a first side of the CdS layer is indirect physical contact with the transparent conductive layer, the firstside being doped with magnesium; and depositing a layer comprising CdTein contact with the CdS layer.
 10. The method of claim 9, wherein CdSlayer includes 1-20% magnesium.
 11. The method of claim 9, wherein thetransparent conductive layer is positioned over a substrate.
 12. Themethod of claim 9, wherein the transparent conductive layer ispositioned over the CdS layer.
 13. The method of claim 9, furthercomprising treating the CdTe layer with cadmium chloride.
 14. The methodof claim 13, further comprising treating the CdTe layer with heat atapproximately 380-450 degrees Celsius.
 15. The method of claim 9,wherein CdS layer is deposited to a thickness of between about 200 toabout 3000 Angstroms.